Patterns.hs

newtype Atom =
  Atom Word64

preview :: Atom -> (Maybe Bool, Word64)
preview x =
  let raw = coerce x
      f = shorten raw
   in if | testBit raw 63 -> (Nothing, f)
         | testBit raw 62 -> (Just False, f)
         | otherwise -> (Just True, f)

review :: (Maybe Bool, Word64) -> Atom
review (b, f) =
  let f = shorten f
   in case b of
        Nothing -> coerce (bit 63 .|. f)
        Just False -> coerce (bit 62 .|. f)
        Just True -> coerce f

-- I want to have a pattern for Present, which unpacks to (Bool, Word64), and a pattern for Absent, which unpacks to just Word64.
-- So basically it needs to fake this:
-- data AtomB = Present (Bool, Word64) | Absent Word64
-- How would I write these?

preview :: s -> Maybe a
review :: a -> s

pattern MyPat :: a -> s
pattern MyPat x <- (preview -> Just x)
  where
    MyPat x = review x